The science of toxicology today has reached a stage where the change from exploration of biological responses to xenobiotics in terms of phenomenology, to mechanistic explanations based on molecular logistics is quite apparent. A conceptual mechanism regarding unity and diversity of stress has been proposed by our laboratory. Generation of free radicals leading to membrane damage and alterations in calcium homeostasis seems to be the common unspecific mechanism in toxic responses which may lead to cell death. Changes in the mitochondrial structure and functions due to chemical or oxidative stress gives mechanistic clues and is an ideal system for studying such a phenomenon. Mitochondrial responses to some chemical agents with respect to its redox equilibrium are presented here.